Since Sir Henry Bessemer first patented the twin roll method for the production of steel sheet directly from a liquid melt over 150 years ago, a number of alternate methods of steel production have been developed. Until the 1950's, ingot slab production was the standard practice where steel was poured into stationary molds or casks. Starting in the late 1950's, conventional slab casting through continuous casting was developed as a new route to improve yield, quality, and productivity in the production of steel. It is used to produce semifinished billet, bloom, or slab for subsequent rolling in finishing mills. In 1989, another steel manufacturing process was developed called thin slab casting which was first implemented by Nucor Steel. The process has allowed the production of steel slabs which are typically thinner than those produced by continuous casting. In addition, the process has been cited as one of the two most important developments of the 20th Century. In 1998, the twin roll strip casting process (i.e. Castrip®) was developed by Nucor Steel. In the strip casting process, molten steel is poured into a smooth sheet in one step at the desired thickness without the need for subsequent and expensive rolling operations. This is achieved by directing liquid steel through nozzles which are aimed between the gaps of two 500 mm spinning copper alloy casting rolls.
Conventional steel alloys solidify by what may be termed conventional liquid solid transformation routes. By this route, generally a small amount of liquid undercooling may be achieved prior to nucleation, resulting in the formation of coarse structure, due to rapid diffusion at elevated temperatures. Growth of corresponding crystals occurs in a superheated liquid melt, resulting in conventional growth modes such as dendritic or cellular growth. While theoretically, any metallic element or alloy may form a glass, conventional steels may not form glasses under normal solidification conditions as the critical cooling rates for metallic glass formation of conventional steels may be extremely high and generally in the range of 106 to 109 K/s.
In such a manner, conventional steel processes are designed to cover the challenges in solidification of existing steel alloys but are not designed for the particular challenges and technical hurdles found in solidifying glass forming steels. For example the twin roll process may work well for conventional plain carbon steel. This may be because the primary goal is to solidify the material while the material passes through the rolls; maximizing the total amount of heat removal may only be a minor or secondary goal. Since conventional steel alloys may undergo cooling to a few tens of degrees sufficient to solidify the melt, not much heat has to be removed before the solidification occurs.
However, in glass forming systems, in order to avoid crystallization, the undercooling may be from the melting point down to room temperature. It should also be appreciated that a sufficient level of undercooling may be from the melting point down to the glass transition temperature (Tg), since below the fictive glass transition temperature diffusion may be so slow that the effective kinetics allows almost a total cooling rate independence. Thus, as discussed above, the total undercooling necessary in conventional steels may generally be ≦50° C. but for glass forming steels, the total undercooling may be much greater and may typically be in the 500° C. to 1000° C., range depending on the alloy chemistry. Such undercooling has limited the maximum thickness of the amorphous structures achievable. Particularly as the amorphous structures solidify they may tend to have low thermal conductivity hindering the removal of thermal energy from the interior of the structure. Thus, solidification behavior in glass forming metallic alloys may be significantly different than what is found in conventional metal solidification.